Super boring answer. When chatting back and forth with the DSP engineers, the casual word we user for the block of processing code that adds the decay in the reverb is "Tank". There's a lot more in the reverb, but the tank, and how the Tank is set with its internal parameters, gives the strongest voice character to the reverb. So in the Double Tank I took the tanks from two existing reverbs, revoiced the tanks slightly, and made a reverb with two tanks in series. The result is a very dense reverb that I enjoyed. It's not based on anything specific or that already exists. It was just a crazy idea that I could hammer into a good-sounding reverb.